The effect of tumor-bearing on host tumor immunity will be studied in vivo and in vitro. The role of various "blocking factors" in the induction and reversal of tumor-mediated suppression of immune reactivity will be examined using the chromium-51 release assay for cytotoxicity. Experiments are proposed to determine the nature and basic characteristics of the factors responsible for the impairment of lymphocyte reactivity in tumor-bearing animals. In addition, the relationship of impaired lymphocyte reactivity, lymphocyte sequestration, and tumor burden will be studied.